Cutting tool coated with diamond

ABSTRACT

A cutting tool coated with diamond in which diamond is coated as a film on a substrate having a step 15 to 500 μm in height in the vicinity of a cutting edge on the rake surface and/or the flank surface. Only the diamond film in the vicinity of the cutting edge can be polished first, without polishing the film of the diamond formed centrally of the rake surface and/or the flank surface. This enables the maximum effect (sharpening of the cutting edge and prevention of welding formation at cutting edge) to be realized with the necessary minimum polishing (with a decreased polishing removal), without setting special conditions and which is suited as a finishing tool.

FIELD OF THE INVENTION

This invention relates to a cutting tool coated with diamond. Moreparticularly, it relates to a throw-away cutting tool coated with a filmof diamond and which is used as a cutting bit or insert, a millingcutter, an end mill or a drill.

[Definition]

The term “diamond” or “diamond film” used herein denotes not only purecrystalline diamond but what is synthesized as a diamond film, i.e.,embracing diamond-like carbon, graphite or amorphous carbon, or amixture thereof. Such diamond may be sometimes calleddiamond-or-the-like or pseudo-diamond.

RELATED ART

If diamond is applied in a film shape on a cemented carbide substrate, acutting edge is rounded due to the presence of the diamond film. Therounding becomes more pronounced the thicker the diamond film, with theresult that the tool is lowered in cutting performance. Moreover, sinceirregularities due to the diamond crystals are present on the as-coateddiamond film surface, the cutting chips are hardly removed to produce alarge cutting resistance. In addition, welding tends to be produced toaffect the durability of the cutting tool.

The routine practice for coping with this problem has been to grind thediamond film surface, after diamond coating, to sharpen the cuttingedge, or to smooth the film surface to prevent welding.

For sharpening the cutting edge of the cutting tool, there is disclosedin JP Patent Kokai JP-A-3-67602 a technique of polishing both the rakesurface and the flank surface, following diamond coating, to sharpen thecutting edge. On the other hand, there is disclosed in JP Patent KokaiJP-A-4-201102 a technique of polishing the rake surface smooth followingdiamond coating, and of polishing the diamond film off from the flanksurface to sharpen the cutting edge.

Among known techniques used for polishing natural or artificial singlecrystal diamond, there are techniques such as co-grinding with a diamondwheel having diamond abrasive grains or diamond particles embeddedtherein and scarfing polishing employing a cast iron plate. However, if,in these methods, the co-grinding pressure is raised, or the tool iscontacted with a diamond wheel rotating at an elevated speed, the coateddiamond film tends to be peeled off.

Thus, in the JP Patent Kokai JP-A-62-41800, JP Patent KokaiJP-A-63-144940 and in the JP Patent Kokai JP-A-63-57160, such a methodis proposed in which, for improving the polishing efficiency, thediamond surface of the substrate is contacted in a non-oxidizingatmosphere with a heated metal surface to graphitize the diamond on thefilm surface to remove the graphitized diamond.

As other smoothing methods, there is disclosed in JP Patent KokaiJP-A-4-331800 a method of sputtering the diamond film surface with anion beam. There has also been reported in Yoshikawa et al., Processingor CVD Diamond Films by YAG Laser: Journal of Precision Engineering, 55,12(1990) 2256, a method of machining the diamond film by the laserlight.

There is also proposed, in JP Patent Kokai JP-A-3-190605, a method ofpolishing a portion of a diamond film on the substrate of the cuttingtool which is formed on the cutting edge and in a portion extending fromthe cutting edge to a portion of the flank surface, using a brushcarrying deposited polishing abrasive grains.

In connection with the shape or the structure of stepped portions on therake surface of the tool cutting, there is shown in JP Patent KokaiJP-A-7-60509 such a shape or structure in which triangular-shaped steps(convexed portions) are provided at the four corners of thesquare-shaped substrate, cemented carbide films, such as diamond films,are applied to these portions, and only the cemented carbide filmportions at the corner portions are polished to a smooth finish ascompared with the surface roughness encountered at the time of filmdeposition (see FIGS. 7 and 10).

SUMMARY OF THE DISCLOSURE

However, in the course of the investigations toward the presentinvention the following problems have been encountered. Namely, theproblems of the above-described conventional techniques are hereinafterscrutinized.

The techniques proposed in the above-mentioned JP Patent KokaiJP-A-3-67602 or in the JP Patent Kokai JP-A-4-201102 suffer a problemthat the polishing operation is time-consuming because the entire rakesurface needs to be polished. On the other hand, if the diamond film onthe flank surface is polished off, the state of an edge portion is suchthat the diamond film and the boundary surface of the substrate areexposed to the exterior, so that the tool cannot withstand heavy dutycutting such as on aluminum alloys and hence the film tends to be peeledoff.

The method of graphitizing and removing diamond, as disclosed in theabove-mentioned JP Patent Kokai JP-A-62-41800, suffers problems that themethod requires heating to elevated temperatures, and is necessarily ofthe batch type because the processing is carried out under thenon-oxidizing atmosphere, and that a special equipment is required forassuring safety if hydrogen is used as an atmosphere, so that the methodis not suitable for industrial production.

On the other hand, the technique of sputtering or machining of thediamond film surface by an ion beam or the laser suffers a problem thatthe apparatus is complex in structure and low in mass producibility suchthat the technique is not suitable for industrial production.

With the method of polishing the cutting edge of the diamond film usinga brush carrying deposited grinding grains, as proposed in JP PatentKokai JP-A-3-190605, the major portion of the irregularities ascribableto the inherent shape of the diamond grains in the vicinity of the edgeare removed on polishing, so that it is possible to prevent the loweringof the weld as well as to prevent the work surface from becoming rougheddue to transcription of the irregularities ascribable to the inherentshape of the diamond grains. However, since the polishing is carried outby a brush, a rounded tool cutting portion cannot be sufficientlyremoved due to dulling by polishing, with the result that sharp edgesusable for finishing machining cannot be obtained. There is also aproblem that technical difficulties are encountered in polishing onlythe vicinity of the edge portions by a simple well-known technique otherthan polishing with a brush. For example it is difficult to bring thediamond wheel into partial grinding contact with the work to highprecision, such that the film coating tends to be peeled off.

Moreover, since the diamond film is inferior in general in tightadhesion to the substrate, in particular to the cemented carbidesubstrate, researches are proceeding for improving the adhesion of thediamond film. The present inventors have proposed a cutting tool with acoating of diamond which, by surface-processing the substrate by heattreatment (JP Patent Kokai JP-A-7-90321, corresponding to U.S. Pat. No.5,858,480 and U.S. Pat. No. 5,725,932 and EP 0627498A) or byelectrolytic etching (JP Patent Kokai JP-A-10-310494, corresponding toU.S. patent application Ser. No. 08/977,972, pending, and EP 0864688A),is endowed with sufficient adhesion to withstand heavy-duty cutting.

However, there are occasions where the substrate becomes deformed bysurface processing of the substrate to affect the polishing operation onthe coated diamond film. Specifically, there are occasions where a midportion of the cutting surface of the substrate becomes convexed(warped) upwards due to heat treatment such that the mid portion of thecutting surface becomes higher than the cutting edge by 5 to 30 μm. Inthe case of electrolytic etching, the speed of electrolytic etching atthe cutting edge portion becomes faster than at other portions due to ahigher current density, thus increasing the amount of the substrateremoved from the cutting edge portion. This occasionally gives the shapeof the tool in which the cutting edge portion is lower in height thanthe mid portion of the rake surface by to 5 to 15 μm.

In any of the above-described techniques, the mid portion of the rakesurface of the substrate becomes higher in height than the cutting edgeportion, so that, if the polishing is performed after coating thediamond film for sharpening the cutting edge, the convexed mid portionof the rake surface is initially contacted with the polishing wheel,however, the cutting edge, which is to be polished, is hardly polished.

If the diamond film is polished, using a metallic material heated toraise the polishing speed, as disclosed in the aforementioned JP PatentKokai JP-A-62-41800, prolonged machining is required for polishing up tothe cutting edge sufficiently. Also, if the substrate has warping, themid portion of the rake surface needs to be polished in addition to theportion that inherently needs to be polished, thus lowering theefficiency. Also, depending on the degree of the warping and thethickness of the diamond coating film, the diamond film is removedexcessively to expose the substrate to the exterior.

In a cutting tool in which the triangular-shaped steps (convex portions)are provided at respective corners of the square-shaped rake surface, asdisclosed in JP Patent Kokai JP-A-7-60509, as shown in FIG. 10, there isa risk of the polishing quantity being increased to lower the polishingefficiency if, when the rake surface undergoes convexed warping, accountis not taken of the height of the steps of the rake surface, diagonallengths of the entire rake surface or of a width 105 of the step of therake surface in the diagonal direction.

It is an object of the present invention to provide a cutting tool fordiamond in which the cutting edge can be sharpened by polishing for onlya short time and which can be conveniently used in particular as a toolfor the finishing machining.

A substrate according to the present invention has its portion raised ata pre-set width along a cutting edge on a rake surface and/or on a flanksurface. If diamond is applied as a film on a substrate having thisshape, the film of diamond (may be termed as “pseudo-diamond) in thevicinity of the cutting edge as an effective cutting portion has aheight larger than the remaining portions. The result is that, if thevicinity of the cutting edge is polished for sharpening the cutting edgeor smoothing the effective cutting edge face, it becomes possible topreferentially polish the film of diamond in the vicinity of the cuttingedge, without first polishing the film of diamond formed centrally ofthe rake surface and/or the flank surface, thus realizing the maximumeffect (i.e., sharpening of the cutting edge and prevention of weldedcutting chips) with the necessary minimum polishing (with a decreasedremoval by polishing). The result is that the sharpening of the cuttingedge and smoothing of the effective rake surface can be easily realizedin short time by polishing by scarfing (polishing over a wide area)suited to industrial production.

The substrate according to the present invention is useful as asubstrate subjected to pre-processing (surface modification), such asheat treatment, for improving tight adhesion between the film of diamondwith the substrate. The reason is as follows: There are occasions wherethis heat treatment deforms the substrate or causes warping such thatthe rake surface and/or the flank surface is raised at its centerportion. If, due to this warping, the film of diamond at the center ofthe rake surface and/or the flank surface is higher in level than thediamond film in the vicinity of the cutting edge, it is necessary topolish the diamond film at the mid portion excessively in an amountcorresponding to the warp. Since the diamond is the hardest of variousmaterials, this difference in the polishing amount gives rise to anappreciably prolonged period of time for machining. According to thepresent invention, the removal by polishing for this portion that neednot be polished can be decreased significantly or even reduced to zero.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate the shape of the step of a cutting toolembodying the present invention, wherein

FIG. 1A shows a triangular-shaped step and

FIG. 1B shows an L-shaped step.

FIGS. 2A to 2F illustrate the shape of steps of cutting tools embodyingthe present invention and specifically are to plan views of various steppatterns.

FIGS. 3A and 3B illustrate the shape of a cutting tool according to anExample A of the present invention, wherein

FIG. 3A is a top plan view and

FIG. 3B is an end side view.

FIGS. 4A and 4B illustrate the shape of a cutting tool according to anExample B of the present invention, wherein

FIG. 4A is a top plan view and

FIG. 4B is an end side view.

FIGS. 5A and 5B illustrate the shape of another cutting tool accordingto the Example B of the present invention, wherein

FIG. 5A is a top plan view and

FIG. 5B is an end side view.

FIGS. 6A and 6B illustrate the shape of a cutting tool according to anExample C of the present invention, wherein

FIG. 6A is a top plan view and

FIG. 6B is an end side view.

FIGS. 7A and 7B illustrate the shape of a cutting tool according to aComparative Example, wherein

FIG. 7A is a top plan view and

FIG. 7B is an end side view.

FIG. 8 shows an example of a case wherein a long cutting edge isrequired at the time of cutting in the cutting tool.

FIG. 9 shows the relation between the step height and the amount of warpin pre-set samples and the length of a diagonal line of a substrate aswell as the results of evaluation thereof.

FIGS. 10A to 10C illustrate a conventional cutting tool in which atriangular step is formed at each corner of a substrate, wherein

FIG. 10A is a top plan view showing a step having a small cutting edge,

FIG. 10B is a top plan view of a step having a large cutting bit and

FIG. 10C is a cross-sectional view taken along a diagonal line.

PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the present invention are now explained indetail.

In the present embodiments of the present invention, the step height isdesigned, depending on the predicted quantity of warp, which isapproximately 50 μm or less, so that, if the pre-coating substrate issubjected to warping, such as by pre-processing (surface modification)executed for improving adhesion of the diamond, the diamond film coatingthe vicinity of the cutting edge is higher in level than the diamondfilm that coats the mid portion of the substrate. The step height ispreferably 15 to 500 μm, more preferably 20 to 300 μm and mostpreferably 50 to 200 μm. This prevents the step difference fromdisappearing by pre-processing, while preventing the diamond film frombecoming peeled off at the step difference portion.

In the preferred embodiments of the present invention, the step width orthe step height is set depending on the polishing efficiency or theconfiguration of a cutting edge portion, such as cutting edge widthand/or depth of cut.

In the preferred embodiments of the present invention, the areal ratio(%) of the raised portion of the substrate to its entire surface (areaof the raised portion/area of the entire basis surface), is 1.5% to 75%.This assures a sufficient width of the raised step portion in thevicinity of the cutting edge (cutting edge portion), while preventingwelding and improving the durability and the polishing efficiency. Moredesirably, the areal ratio is set to 3% to 65% and, most desirably, itis set to 5% to 50%.

In a preferred form of the present invention, the steps are provided forextending along the cutting edge of the rake surface or the flanksurface. In another preferred form, the steps are provided on at leastthe rake surface among the cutting surfaces in the cutting edge portion.The step boundary may be set to be upstanding (steep cliff) or may becontinuously changing in height.

For obtaining the stepped substrate, it is possible to prepare a steppedmolded article at the outset using a stepped metallic mold incompression molding a powder mixture of a starting cemented carbidematerial. It is also possible to prepare a blank of cemented carbide andsubsequently machine the blank to a stepped shape using known machiningtechniques, such as electrical discharge machining or sandblasting.

In the preferred embodiments of the present invention, cemented carbidealloys, such as WC-based, TiC-based or TaC based cemented carbidealloys, cermet-based or ceramic-based materials, are used as thesubstrate. As a film-shaped coating for the substrate, diamond, cBN,mixtures (composite layers) thereof, or an extremely hard material, isused.

In the preferred embodiments of the present invention, the substrate issurface-treated by heat treatment or electrolytic etching, prior topolishing the diamond film, in order to raise the adhesion between thesubstrate and the diamond film. Among base materials suited to thissurface processing method, there are cemented carbides, such as WC groupbased cemented carbides, cermet and ceramics. In the JP Patent KokaiJP-A-7-90321 and in JP Patent Kokai JP-A-10-310494 by the presentinventors, there are contained descriptions on heat treatment andelectrolytic etching, to which reference is to be had as necessary inthe present specification, the entire disclosure thereof beingincorporated herein by reference thereto.

The substrate of the present invention is suitably applied when thesubstrate undergoes slight deformation in shape due to pre-processing,performed for improving the adhesion, such that μm-level warping occursto produce a convexed mid portion in the same plane, with edge portionsin the same plane being recessed on the μm level as compared to the midportion.

A preferred method for forming a diamond film and pre-processing(surface modification) therefor are now explained.

[Damaging Processing]

After the pre-processing for the substrate, such as after electrolyticetching, removal processing by an acid or masking with an intermediatecoating layer may be carried out for suppressing the effect of a bondingphase component present on the substrate surface. For improving thequantity of generation of diamond nuclei, such a damaging processing canbe performed in which the substrate is immersed for ultrasonicprocessing in an acetone solution having dispersed therein fine diamondparticles having a mean particle size of 5 to 10 μm.

[Method for Diamond Synthesis]

For synthesis of diamond to the processed substrate, any suitable knownmethods for gas-phase diamond synthesis, such as the CVD or PVD methods,may be employed. In particular, a hot-filament method, a RF plasma CVDmethod or a micro-wave plasma CVD method, is preferred.

[Gases as Starting Material for Diamond]

As a starting material for diamond, any suitable material, such ascarbides (for example methane, ethane or propane), alcohols (such asethanol), CO or CO₂, may be used. These starting materials may be usedalone or as a mixture and may also be diluted with a hydrogen gas or aninert gas.

[Method for Polishing Diamond Films]

The diamond film, applied on the substrate, can be polished using anyoptional polishing method. Preferably, such a method may be used whichconsists in polishing by scarfing, suited for industrial production, orin polishing a diamond surface by thrusting a diamond-coated insertagainst a rotating diamond wheel carrying diamond abrasive grains at apressure insufficient to peel off the coated diamond coating.

In a present embodiment of the present invention, the followingsubstrate material is heat-treated in the following manner, prior todiamond coating, for improving adhesion between the diamond film and thesubstrate.

As the substrate material, a WC-based cemented carbide, mainly composedof WC, and also containing other components, preferably Ti, with orwithout Ta, and at least one of Co and Ni, as a bonding phase. Apreferred composition of the WC-based cemented carbide comprises 0.2 to20 wt %, preferably 0.5 to 10 wt % and more preferably 1 to 5 wt %, ofTi, or Ti and Ta, as carbide, 2 to 15 wt %, preferably 3 to 10 wt %,more preferably 4 to 7 wt %, of at least one of Co and Ni, and at leastone of a W—Ti—C solid solution (β-phase) and a W—Ti—Ta—C solid solution(βt-phase). A preferred mean crustal grain size of the β-phase and theβt-phase is 0.5 to 10 μm and preferably 1 to 5 μm.

If the content of Ti as carbide is less than 0.2 wt %, a N-containingirregular surface layer is not likely to be produced due to heattreatment, while the surface layer is liable to be detached followingheat treatment. The reason the surface layer is liable to be detached isthat the major portion of the Ti component migrates to a surface regionby heat treatment to form a W—Ti—C—N solid solution (β(N) phase) on thesurface to separate the Ti component and other alloy components toaffect the fitting state. If the amount of Ti as carbide exceeds 20 wt%, the substrate is already brittle prior to heat treatment, while thethermal expansion coefficient of the substrate is increased and thedifferential between the thermal expansion coefficient of the substrateand that of diamond is increased to produce a shearing stress on aninterface between the substrate and the diamond film during coolingfollowing diamond coating to give rise to film detachment (peeling-off).

This also accounts for the preferred upper limit value of 20 wt % of thecemented carbide in case Ta is contained in addition to Ti.

Meanwhile, Ta may be replaced in part or in its entirety by at least oneof V, Zr, Nb and Hf, insofar as it does not affect the aforementionedheat treatment. It is noted that the WC-based cemented carbide, obtainedon densely sintering powders of WC, TiC, TaC and/or Co by a powdermetallurgical method, is lowered in strength if the carbide crystalphase undergoes grain growth during sintering. Thus, at least one of Crand Mo, suppressor of grain growth during sintering, can be contained,usually as carbides, within a range not affecting heat treatment in thepresent invention.

If the content of at least one of Co and Ni, as a bonding phase, is lessthan 2 wt %, it is difficult to achieve densification by sintering atthe time of manufacturing the WC-based cemented carbide, such thatcharacteristics required of the substrate, such as strength, would fallshort. If the above content exceeds 15 wt. %, these components tend toappear (or migrate) on the substrate surface at the time of heattreatment or of forming the diamond film coating, thus incidentallyaffecting the formation of the diamond film. On the other hand, thethermal expansion coefficient differential from that of the diamond filmmay occasionally become larger to give rise to film detachment.

If the average particle size of the β-phase or the βt-phase is less than0.5 μm, there are occasions where irregularities formed on theN-containing surface layer formed after the heat treatment becomesmaller or sufficient fitting (or mechanical engagement) between theN-containing surface layer and the WC-based cemented carbide inner layercannot be achieved. If the above-mentioned average particle size exceeds10 μm, there are occasions where above-mentioned fitting becomesinsufficient, or the strength as the cemented carbide prior tp heattreatment cannot be realized.

Meanwhile, in the case of a N-containing cemented carbide or cermetcontaining β(N) phase from the outset, due to sintering after additionof N-containing powders, such as a TiN or TiC—TiN solid solution or dueto sintering in N atmosphere containing nitrogen atoms, there areoccasions where the irregularities are not produced with ease even onheat treatment, or where the state of the irregularities becomesdifficult to control by the heat-treatment atmosphere.

For correctly controlling the N₂ content in the heat treatmentatmosphere for the WC-based cemented carbide, it is preferred toconstruct an oven used for heat treatment from refractories notaffecting the N₂ content in the atmosphere.

The preferred heat treatment temperature for the WC-based cementedcarbide is 1350 to 1450 degrees C., with the lower limit temperaturediffering with the carbon quantity in the alloy or with the Co/Niquantity ratio.

The heat treatment time affects the degree of the irregularities on theN-containing surface layer most appreciably. By adjusting this factor,it becomes possible to form a N-containing surface layer having anydesired irregularities. For producing a N-containing layer efficientlyin stability, the heat treatment temperature or the amount of N₂ in theatmosphere may be adjusted to set the heat treatment time preferably to0.5 to 5 hours.

The atmosphere for heat treatment contains N₂ in an amount of 0.05 to 5vol. % at ambient pressure. Preferably, N₂ is contained in an amount of0.5 to 3 vol. %, with the balance being an inert gas, such as Ar.

Alternatively, re-heating under an inert atmosphere, such as argon, maybe carried out after formation of the N-containing irregular surfacelayer, for releasing N from the surface layer, insofar as this does notaffect film deposition properties of the surface layer.

Still alternatively, as a method for achieving the meritorious effectequivalent to that of the above-mentioned re-heating, that is the effectof not having N contained on the uppermost surface, a hard coating ofTiC may be applied by a well-known method, such as CVD or PVD, to athickness which will not significantly affect the surface shape (ornature) of the irregular surface layer.

As a method for coating diamond, the so-called CVD method of contactingan excited gas mixture of a carbon source gas and a hydrogen gas may beused. In particular, a micro-wave plasma CVD method is preferably usedas means for controlling the synthesis conditions to high accuracy.

As a preferred embodiment of the present invention, the followingsubstrate material is used and etched in the following manner, prior todiamond coating, in order to improve adhesion to the diamond film.

As a substrate material, a cemented carbide substrate is composed mainlyof tungsten carbide (WC) and also containing one or more selected fromcarbides, nitrides and carbonitrides of Ti, Ta, Nb and V in an amount of0.3 to 10 wt % and preferably in an amount of 0.5 to 10 wt % as aconverted weight as carbide, and Co and/or Ni in an amount of 2 to 10 wt% as a sum.

This substrate is of the so-called cemented carbide, mainly composed ofWC, and is optimum in a bonding strength of the diamond film, owing tothe fact that there is contained therein a hard phase containing one ormore selected from carbides, nitrides and carbonitrides of Ti, Ta, Nband V. The hard phase may also be in the form of a W-containing solidsolution. Specifically, when forming irregularities on the substratesurface by electrolytic etching, the rate of removal of the hard phasemainly composed of carbides, nitrides and carbonitrides of Ti, Ta, Nband V is slower than that of the hard phase mainly composed of WC, sothat irregularities of a preferred form are formed on theelectrolytically etched surface of the cemented carbide substratecontaining the above compounds. If the sum of the amount of thecarbides, nitrides and carbonitrides of Ti, Ta, Nb and V in thesubstrate, calculated as carbide, is less than 0.5 wt %, in particularthan 0.3 wt %, the effect of having these components contained in thesubstrate is diminished. Conversely, with the above amount exceeding 15wt %, the bonding strength tends to cease be improved further.

The desirable average particle size of the hard phase in the cementedcarbide, mainly composed of the carbides, nitrides and carbonitrides ofTi, Ta, Nb and V, depends on the magnitude of the irregularities formedon the substrate surface. For obtaining the bonding strength of thelarge diamond film, the average crystal grain size of these compounds ispreferably 0.3 to 5 μm, preferably 0.5 to 2 μm and more preferably 1 to2 μm.

If the content of Co and/or Ni in the substrate is larger, the diamondfilm is lowered in bonding strength, whereas, if the content is smaller,the diamond film becomes lower in mechanical properties, such asstrength. Therefore, the content is adjusted depending on theapplication of the cutting tool. It is noted that the combined amount ispreferably 2 to 10 wt % and in particular 3 to 6 wt %, since then thecutting tool can be used for a wide variety of the fields ofapplications.

In the above-described cutting tool coated with diamond, diamond ispreferably coated as a film on the substrate having portions raised at apre-set width to a pre-set height along a cutting edge on the rakesurface and/or the flank surface.

Also, in the cutting tool coated with diamond, the raised portion ispreferally of a step of 15 to 500 μm.

Also, in the cutting tool coated with diamond, the rake surface in itsentirety is substantially in the form of a polygon, such as square orparallelogram. The height of the step on the rake surface h(μm) ispreferably related to a diagonal length W(mm) of the entire rake surfaceby h≧2 W+4.

In the cutting tool coated with diamond, the shape of the step of therake surface is preferably a partial V- or L-shape, having each corneras an apex point. Referring to FIG. 1, if the step shape is changed froma triangular shape FIG. 1A to a V- or an L-shape FIG. 1B, the width ofthe step in the diagonal direction of the rake surface 1 can bedecreased from w2 to w3 as the length w1 of the blade of the rakesurface step 2 is maintained. That is, there is no risk of the stepwidth increasing along the diagonal line of the rake surface as occurswhen the triangular shape of the step of the rake surface is maintainedand the cutting edge is increased in length, as shown in FIGS. 10A and10B. This decreases the amount of polishing to increase the polishingefficiency further. Referring to FIG. 2, if the length of the cuttingedge is increased from FIG. 2A to FIG. 2B, the polishing efficiency isnot lowered, because the step of the rake surface in the diagonaldirection remains constant. Similar operations and effects can beobtained for the patterns as shown in FIGS. 2D and 2F.

Also, in the cutting tool coated with diamond, the shape of the rakesurface step is preferably annular or frame-shaped along substantiallythe entire periphery. Referring to FIGS. 2A-2F, the V-shaped steps asshown in FIG. 2B are preferably interconnected to form an annular stepas shown in FIG. 2C. If the rake surface step is annular orframe-shaped, the cutting edge length can be maximized with respect tothe size of the cutting tool. Moreover, as for manufacturing, molding bya biaxial molding device is facilitated to reduce the number of themachining steps. Similar operations and results can be obtained with apattern shown in FIG. 2E.

EXAMPLE

For clarifying the above-described embodiment of the present invention,an example of the present invention is hereinafter explained byreferring to the drawings.

Example A

As starting powder material, WC powders, with an average particle sizeof 2 μm, powders of TiC—WC solid solution, TaC powders and Co, with anaverage particle size of 1 μm, were assorted in proportions shown inTable 1 (compositions A to D). The respective powder mixtures werepress-molded at a pressure of 1.5 ton/cm² (approximately 1.47×10⁴ N/m²).The resulting pressurized powders were fired in vacuum for one hour at1400 to 1450 degrees C. to prepare sintered products havingapproximately the same compositions as the compositions indicated inTable 1. The surfaces of the sintered products were ground to preparesubstrates which serve as a base shaped in accordance with the ISOstandard SPGN 120304 to 120320.

Also, substrates serving as a base shaped in accordance with SPGN 120304were prepared.

TABLE 1 Composition W and sintering symbol TiC TaC Co impuritiestemperature A 0.5 — 5 balance 1400° C. B 3 — 5 balance 1450° C. C 3 2 5balance 1450° C. D 7 5 7 balance 1450° C.

The relation between the length of a diagonal line of the rake surfaceof the substrate and the warping is explained. If the amount of warp islarger than the step height, the diamond film at the mid portion of therake surface is contacted with the polishing wheel during polishing ofthe rake surface. Therefore, the provision of the step is meaningless.On the other hand, the longer the length of the diagonal line of thesubstrate, the larger is the warping. In light of the foregoing, testswere conducted on the relation between variations in the warping due tothe length of the diagonal line and the step height. The results of thetest on the relation between variations in the warping due to the lengthof the diagonal line and the step height are shown in Table 2. In thistable, if the amount of warp is smaller than the step height, machiningis presented to be possible, whereas, if the amount of warp is largerthan the step height, machining is presented to be impossible.

TABLE 2 step step height > height < amount amount length of warp of warpof (flank (flank dia- surface surface num- gonal step polish- polishingber machina- substrate line height ing not of bility shape (mm) (μm)possible) possible) tests (%) SPGN120304 ca.16 40 10 0 10 100 SPGN120304ca.16 30 31 19 50 62 SPGN120304 ca.16 20 12 38 50 24 SPGN120304 ca.16 100 10 10 0 CPGN120304 ca.18 50 10 0 10 100 CPGN120304 ca.18 40 28 2 30 93CPGN120304 ca.18 30 18 12 30 60 CPGN120304 ca.18 20 5 25 30 17CPGN120304 ca.18 10 0 30 10 0

Using an electric discharge machine, shown in FIGS. 3A and 3B, a recessshown in FIGS. 3A and 3B was formed at a mid portion of the rake surfaceof the substrate 3, so that a step dimensioned as shown in Table 3 willbe formed along the four sides of the rake surface of the substrate.There were also provided a substrate having a narrow step width W, asubstrate having a low step width and a substrate having no step(samples 10 and 11).

TABLE 3 Example A (step shape conforms to FIG. 3) areal flank cut- stepratio of polish- ting initial material width cutting rake surface ingedge depth surface sam- compo- substrate W step height face areapolishing time R of cut roughness ples sition shape (mm) h (mm) (%) time(min) (min) (μm) (mm) Rz (μm) cutting time (min) 1 A SPGN120304 0.5 0.115 30 30 5 0.3 5.2 >100 2 A SPGN120304 0.1 0.05 3 15 30 5 0.1 5.1 >100 3B SPGN120304 2 0.2 55 180 30 <10 0.5 5.5 >100 4 B SPGN120308 1 0.1 30 9030 5 0.2 3.7 >100 5 C SPGN120308 1.5 0.1 42 120 30 5 0.5 4.1 >100 6 BSPGN120308 0.5 0.5 15 30 0.5 5 0.1 3.8 detached in 5 min 7 C SPGN1203081 0.01 30 interrupted* — — — — — 8 B SPGN120308 0.04 0.1 1 5 0.5 5 0.1 —detached at the outset 9 C SPGN120308 3.5 0.1 85 330 0.5 <10 0.33.8 >100 10 C SPGN120308 ISO shape without step 100 360 0.5 5 0.23.8 >100 11 sintered SPGN120308 ISO shape without step — — — <5 0.23.6 >100 diamond 12 C SPGN120308 1 0.1 40 non- — >30 0.2 7.2 >100polishing 13 C SPGN120312 1.4 0.1 40 90 0.5 5 1.0 2.8 detached in 75 min14 C SPGN120312 2 0.2 55 180 0.5 10 1.2 3.1 detached in 75 min 15 CSPGN120320 2 0.2 55 180 0.5 10 1.5 2.2 detached in 50 min 16 DSPGN120320 3 0.4 70 200 0.5 15 2.0 4.3 detached in 50 min 17 CSPGN120304 0.5 0.005 30 interrupted* — — — — — 18 C SPGN120304 0.5 0.0230 15 15 5 0.1 4.8 >100 19 C SPGN120308 1 0.04 30 90 30 5 0.2 3.9 >100*The polishing of samples 7 and 17 were interrupted because the warp ofthe material was higher than the step height such that the diamond filmat a mid portion of the rake surface contacted with an abrasive wheel.

The substrate samples obtained were charged into a carbon casing andheat-treated for three hours at 1 atm (1.013×10⁵N/m²) and at 1350° C.,in an atmosphere of 1 vol. % nitrogen and 99 vol. % argon, using anelectrical furnace, the components of which were exposed to elevatedtemperatures, such as heater or insulating materials, were formedentirely of carbon. The substrate samples, thus processed, were immersedin a solvent in which fine diamond powders, with a mean particle size of10 μm, were floated and dispersed. In this state, the substrate sampleswere ultrasonically processed to activate the entire surface.

The substrate, thus obtained, was installed in a microwave plasma CVDdevice of 2.45 GHz. The electric power was adjusted so that, under thetotal pressure of 50 Torr (about 6666 N/m²), 85 vol % H₂ 15 vol % CO,the substrate temperature of 900° C. will be reached, to generate aplasma, which was maintained for 15 hours to apply the diamond coatingto a film thickness of approximately 30 μm. It was seen that the diamondon the coating film surface was of a particle size of 10 to 15 μm andwas in a state in which polycrystals presenting irregularities with aparticle size of 10 to 15 μm were densely packed together.

The back surface of the diamond-coated substrate was affixed to apolishing holder and polished as its rake surface was applied to a #1000vitrified bond diamond wheel. It was seen that, as shown in Table 3, thepolishing time for the rake surface was appreciably reduced incomparison with the sample 10 having no step in the rake surface.

Similar polishing was carried out on the flank surface to sharpen thecutting edge so that its R will be not larger than 10 μm. Forcomparison, the R value of the cutting edge at a cutting edge of asample 12 not polished in this manner was not less than 30 μm.

Using the cutting inserts (tips), thus obtained, cutting tests wereconducted under the following conditions. The inserts of the presentembodiment exhibited surface roughness and durability equivalent tothose of a sintered diamond tool. It may thus be seen that, according tothe present invention, the inserts for finishing machining having thesame performance can be obtained in a shorter insert polishing time.

Cutting test conditions: evaluation of roughness of surface to beprocessed (surface roughness of the turned outer peripheral surface of acylindrical work)

Turned warkpiece: Al-18% Si alloy approximately 150 mm in diameter andapproximately 200 mm in length

Cutting speed: 800 m/min

Feed: 0.15 mm/rev

Depth of cut: 0.1 to 2.0 mm.

Example B

Using starting powders (see Table 4), adjusted at a mixing ratio of thecomposition B or the composition C of Example A (see Table 1), and apress molding metal die having a convex portion to generate a rakesurface of the substrate having a step, a compacted powdered mass havinga step at a portion corresponding to the rake surface was prepared bypress molding under a pressure of 1.5 ton/cm² (approximately 1.47×10⁴N/m²). This compacted powered mass was sintered by a method which is thesame as Example A to produce a cemented carbide substrate having thebasic shape according to the ISO standard CPGN 120408 with a step shownin FIG. 4 or 5 and with a step on the cuffing edge of the configurationshown in Table 4. Meanwhile, the substrate shown in FIG. 4 has beenformed by an electric discharge machine so that a recess will be presentat the center of the rake surface, so that a step dimensioned as shownin Table 4 will be formed on the four sides of the rhombus-shaped rakesurface. The substrate shown in FIGS. 5A and 5B is formed by forming agroove centrally of the rake surface of the substrate, using a surfacegrinder, so that a step dimensioned as shown in Table 4 will be formedat the two corners of the rhombus-shaped rake surface. There was alsoprovided a substrate having no step (sample 25).

These substrates were subjected to heat treatment, diamond coating andpolishing for sharpening the cutting edge, as Example A. The producedinserts were subjected to the cutting tests, as in Example A, toevaluate surface roughness of the workpieces to confirm that a tool of asmall surface roughness that can be used for cutting can be obtained bythe polishing of a short duration as shown in Table 4.

TABLE 4 Example B (step shape conforms to FIGS. 4 and 5) areal rakeinitial ratio of surface surface material step step cutting polishingflank cutting rough- compo- step width height face time polishing edge Rdepth of ness Rz samples sition shape W (mm) h (mm) area (%) (min) time(min) (μm) cut (mm) (μm) 20 C FIG. 4 0.5 0.1 10 40 30 5 0.3 3.7 21 CFIG. 4 1.6 0.1 30 120 30 <10 0.5 4.2 22 B FIG. 5 1.7 0.1 1.5 10 30 5 0.23.6 23 B FIG. 5 4.4 0.1 10 40 30 5 0.5 4.0 24 B FIG. 5 5.4 0.1 15 60 305 1.0 4.5 25 C no step ISO shape without 100 400 0.5 5 0.2 3.6 step 26 CFIG. 4 0.5  0.015 10 inter- — — — — rupted* *The polishing of sample 26was interrupted because the warp of the material was higher than thestep height such that the diamond film at a mid portion of the rakesurface contacted with an abrasive wheel.

Example C

WC powders with an average particle size of 2 μm, NbC powders with anaverage particle size not larger than 2 μm, TaC powders with an averageparticle size of 1 μm and Co powders with an average particle size of 1μm, were mixed to give a composition 4 wt % (Ta, Nb). C-6 wt % Co-90 wt% WC, and the resulting mixture was molded. The resulting molded productwas fired in vacuum for one hour at 1450° C. and the resulting sinteredproducts were polished to a shape of the ISO standard SPGN120408 toprepare a cemented carbide blank.

The substrates were machined by machining the rake surface and the flanksurface using an electric discharge machine and a surface grinder,respectively, so that blanks will be shaped as shown in FIG. 6 and sothat the step in the vicinity of the cutting edge will be configured asshown in Table 5. Meanwhile, substrates shown in FIG. 6 was prepared byforming a recess centrally of the rake surface of the substrate by anelectric discharge machine to generate a step dimensioned in Table 5along the four sides of the rake surface, and by forming a step in thevicinity of the cutting edge of each rake surface, using an electricdischarge machine. There was also provided a substrate having no step(sample 30). The produced substrates were electrolytically etched, at acurrent density of 0.15 A/cm² and at a maximum removing rate of 0.65μm/min, using a 5% KCl aqueous solution as an electrolytic solution. Onthe surface of the electrolytically etched substrate surface weredeposited products of the electrolytic reaction. These products wereremoved by washing in a 10% aqueous solution of NaOH.

The produced substrates had their surface activated with diamondparticles, as in Example A, and were coated with diamond to a thicknessof 30 μm by the micro-wave plasma CVD method. The rake surface and theflank surface of the diamond-coated substrates were polished, as inExample A. It was found that the substrate could be worked to a cuttingedge R not larger than 10 μm in a shorter time than with the sample 30having no step in the vicinity of the cutting edge, as shown in table 5.

In the cutting tests, similar to that in Example A, it was found thatthe polishing performance which is the same as the insert devoid of thestep (sample 30) shown in Table 5 was achieved.

TABLE 5 Example C (step shape conforms to FIGS. 6) initial rake flanksurface step step step step surface polishing cutting depth rough- widthwidth height h height h polishing time edge R of cut ness Rz samples W(mm) W (mm) (mm) (mm) time (min) (min) (μm) (mm) (μm) 27 0.5 0.5 0.1 0.130 10 5 0.2 3.7 28 1.0 1.0 0.1 0.1 90 20 5 0.3 3.7 29 1.5 1.0 0.1 0.1120 20 10 0.5 4.2 30 ISO shape without step 360 30 5 0.3 3.6

Example D

Using starting powders (see table 6), adjusted to a mixing ratio of thecomposition C (see Table 1) of the Example A, and also using a pressmolding metal die for molding having a convex shaped portion foraffording a step on the rake surface of the substrate, starting powderswere press-molded at a pressure of 1.5 ton/cm² (approximately 1.47×10⁴N/cm²), a compacted powder mass, having a step in a portioncorresponding to the rake surface, was prepared. This compacted powdermass was sintered in the same way as in Example A and its surface waspolished to prepare a cemented carbide substrate of a basic shape of theISO standard SPGN 120304 having a step shaped as shown in FIG. 7(samples 31 to 33: Comparative Examples) or as shown in FIG. 3 (sample34: Example) and a step of the cutting edge shown in Table 6. Meanwhile,in the substrate shown in FIG. 7, an octagonal-shaped recess was formedby an electric discharge machine at the center of the rake surface ofthe substrate so that a step dimensioned as shown in Table 6 will beformed at each of the four corners of a square-shaped substrate roundedat the four corners.

This substrate was subjected to heat treatment, diamond coating andpolishing for sharpening the bit end, as in Example A. Table 6 shows theoperating conditions.

With the sample having steps at the four corners (samples 31 to 33:Comparative Examples), splitting of molding metal die of an upper punchinto two becomes complex to render powder molding difficult. Moreover,the sample cannot cope with a case in which a long cutting edge isrequired for cutting, such as cutting of a sprue projection of a pistonfor an engine having an outer diameter of 110 mm and a length ofapproximately 110 mm (see FIG. 8). On the other hand, the method offorming a step by the post-processing leads to an increased number ofsteps and higher manufacturing costs (see JP Patent Kokai JP-A-7-60509for an insert for a cutting tool having steps at the four corners andthe method of providing a step by the post-processing). Conversely, witha sample having an annular step on its outer rim (sample 34: Example),powders can be molded easily at the time of manufacturing a substrate,while molding by a biaxial molding machine having upper and lowerpunches is facilitated.

TABLE 6 Example D number of areal times of number of ratio of rakemachining times of ma- step step cutting surface flank operationsmachining terial sub- width height face polishing polishing for theoperations state of sam- compo- strate step W h area time time outer forthe head machining ples sition shape shape (mm) (mm) (%) (min) (min)diameter portion operations 31 C SPGN120304 FIG. 7 2.0 0.05 10 20 30≧10  ≧5  initial peeling at step 32 C SPGN120304 FIG. 7 3.0 0.1 22 60 30≧3000 ≧5  peeling occurred at step 33 C SPGN120304 FIG. 7 5.0 0.162 >200 30 15000 ≧5000 peeling of diamond film occurred during machiningof the head portion 34 C SPGN120304 FIG. 3 1.0 0.1 30 90 30 15000 to30000 to burrs produced 20000 35000 in a work due to wear of cutting sothat tool service life end was reached

The relation between the step height, amount of warp and the length of adiagonal line in the substrates in the above Examples is explained. Ifthe amount of warp is larger than the step height, the diamond film atthe center of the rake surface is contacted with the polishing wheel, sothat provision of the step becomes meaningless. Therefore, the amount ofwarp is desirably smaller than the step height. If the length of thediagonal line of the substrate becomes longer, the amount of warpbecomes larger. It is therefore necessary to set the step heightdepending on the length of the diagonal line. Thus, equationsapproximating straight lines which determine the range of the amount ofwarp of the substrate, given the substrate height and the effective stepheight, are set in the following manner:

upper limit (amount of warp: μm)=5/6 (length of diagonal line: mm)+10/3

lower limit (amount of warp: μm)=2 (length of diagonal line: mm)+4

Meanwhile, those samples lying in an area below the upper limit linewere not usable marked as “NG” and those lying in an area above thelower limit were usable marked as “OK”. FIG. 9 shows the relationbetween the step height, amount of warp and the length of the diagonalline of the substrate, as well as the results of evaluation of therespective samples.

Meritorious Effects of the Invention

With the cutting tool according to the present invention, its edge canbe polished in a short time without setting special conditions. Thecutting tool of the present invention cuts satisfactorily, such that,with the use of this cutting tool, the workpiece can be cut to anoptimum machined state. Therefore, the cutting tool according to thepresent invention is used satisfactorily especially as a tool forfinishing machining.

It should be noted that other objects, features and aspects of thepresent invention will become apparent in the entire disclosure and thatmodifications may be done without departing the gist and scope of thepresent invention as disclosed herein and claimed as appended herewith.

Also it should be noted that any combination of the disclosed and/orclaimed elements, matters and/or items may fall under the modificationsaforementioned.

What is claimed is:
 1. A cutting tool coated with diamond, said cuttingtool comprising: a rake surface; a flank surface; and a cutting edgedefined by said rake surface and said flank surface; wherein diamond iscoated as a film on a substrate having a step raised at a pre-set widthalong said cutting edge on a rake surface and a flank surface of acutting edge, said substrate generally defining said rake surface andsaid flank surface; wherein said substrate is pre-processed with respectto the surface thereof in a manner which improves adhesion between saidsubstrate and the film of diamond, to form a convex region on said rakesurface of the substrate resulting from aid pre-processing of thesurface of said substrate, said convex region comprising a deformationin the planarity of the surface thereof; said rake surface and saidflank surface also comprising at least one raised step portion locatedalong said cutting edge; and wherein the height of said step is higherthan the deformed surface of said convex region.
 2. The cutting toolcoated with diamond as defined in claim 1, said step is 40 to 500 μm inheight in the vicinity of the cutting edge on the rake surface or theflank surface.
 3. The cutting tool coated with diamond as defined inclaim 2, wherein the coated film of diamond has a thickness of 10 to 50μm.
 4. The cutting tool coated with diamond as defined in claim 1,wherein a portion of the film of diamond on the rake surface or on theflank surface which has been coated on said step is polished to sharpenat least said cutting edge.
 5. The cutting tool coated with diamond asdefined in claim 4, wherein said step of said rake surface is at leastpartially L- or V-shaped with a corner or corners as an apex or asapices.
 6. The cutting tool coated with diamond as defined in claim 1,wherein the coated film of diamond has a thickness of 10 to 50 μm. 7.The cutting tool coated with diamond as defined in claim 1, wherein thecutting tool is configured as a throw-away insert.
 8. The cutting toolcoated with diamond as defined in claim 1, wherein diamond is coated asa film on a substrate having its portion raised at a pre-set width inthe vicinity of and along a rake surface or a flank surface of a cuttingedge.
 9. The cutting tool coated with diamond as defined in claim 8,wherein said raised portion forms a step of from 40 to 500 μm in height.10. The cutting tool coated with diamond as defined in claim 1, whereinthe rake surface in its entirety is substantially of a square orparallelepiped shape and wherein a height h in μm of said rake surfaceand a length W in mm along a diagonal line of said rake surface in itsentirety satisfy the relationship h≧2W+4.
 11. The cutting tool coatedwith diamond as defined in claim 1, wherein said step of said rakesurface is at least partially L- or V-shaped with a corner or corners asan apex or as apices.
 12. The cutting tool coated with diamond asdefined in claim 1, wherein said step of said rake surface is annular orframe-shaped substantially throughout its entire periphery.
 13. Acutting tool comprising: a substrate, and a film of diamond coated onsaid substrate, said substrate comprising a rake surface; a flanksurface; an edge defined by said rake surface and said flank surface;said rake surface comprising a convex region resulting frompre-processing of said substrate to improve adhesion of said diamondfilm; said rake surface also comprising at least one raised step locatedalong said edge, wherein the height of said step is greater than theheight of said convex region, and said flank surface comprising a raisedstep located along said edge.